1. Field of the Invention
The present invention is related to a charging/discharging control unit used for a lithium secondary battery.
2. Description of the Related Art
In view of environmental aspects, specific attentions have been paid to lithium secondary batteries functioning as electric vehicle-purpose batteries and electronic power storage-purpose batteries. In their utilization fields, high energy density and long lifetime are required. As to the long lifetime, 1,000, or more cycles of charging/discharging repetitions are required. In conventional lithium secondary batteries, graphite materials have been employed for negative electrodes thereof in order to realize a high energy density. However, the lithium secondary batteries in which the graphite materials have been employed for the negative electrode thereof have such a problem that cycle lifetime thereof becomes short.
Conventionally, the below-mentioned technical ideas are known with respect to such lithium secondary batteries that graphite materials have been employed for negative electrodes thereof: That is, in the above-descried lithium batteries, by setting discharge termination potentials of the negative electrodes lower than or equal to “0.5 V vs. Li/Li+”, it is possible to avoid a rapid volume change of the negative electrode, which occurs in the last stages of battery discharging operation, and it is therefore possible to improve cycle lifetimes of the above-described conventional lithium secondary batteries (refer to, for instance, JP-A-2005-294028). However, in JP-A-2005-294028, in order to set the discharge termination potential of the negative electrode to be lower than or equal to “0.5 V vs. Li/Li+”, lithium must be previously doped in the negative active material. In order to dope lithium to the negative active material, complex manufacturing steps are necessarily required, since metal lithium foils are assembled in the lithium secondary battery. Also, another negative active material capable of accepting lithium from the positive electrode is required in addition to the lithium which has been previously doped. As a result, there exists such a problem that a total utilization amount of the negative active materials is increased.